Abstract
A re-examination of the El Dragón vein-type Se deposit using polarized light microscopy and scanning-electron microscopy combined with electron-microprobe analyses revealed the following results: the detection of (a) petříčekite, krut’aite and penroseite close to endmember composition; (b) a yet unknown mineral of the ideal composition CuNi2Se4; (c) intermediate members of the vaesite-pyrite solid-solution series; and (d) a mineral with a composition intermediate between athabascaite and its yet unknown S-equivalent, Cu4S5. Triggered by volcanic-hydrothermal activities around the Porco caldera, formation of the mineralization involved five episodes of fault formation and re-opening, fluid-mediated metal mobilization, metalliferous fluid infiltration, and mineral precipitation, re-deposition, and alteration that probably extended from 12 Ma until today. The origin of the Se-vein was accomplished by the multiple injection of highly oxidized, metal-rich fluids into a fault at the interface between black shale and siltstone. These low-T solutions (estimated max. temperature 100 °C, max. pressure 1 bar) possessed high to exceptionally Se fugacities (log fSe2 fluctuating between of −14.5 and −11.2) that prevailed for most of the evolution of the deposit, only interrupted once, during the episode of deposition of sulfides of Cu and Fe and resulting partial alteration of the pre-existing selenides. Formation of end-member krut’aite and native selenium implies a minimum log fSe2 of −11.2 at the final stage of vein formation. The likely source of Se and the accompanying metals (Cu, Ag, Ni, Co, Au, Pb, Hg, Cd, Fe and Bi) is the neighboring Kupferschiefer-type (possibly Devonian) black shale rich in framboidal pyrite, Cu-sulfide aggregates, and organic matter. Deposition of Se-minerals occurred where the oxidized metal-bearing solutions became in contact with a reductant, associated with the reduction of mobile selenate to immobile selenide or elemental Se.
Highlights
Research on selenium minerals conducted in the past few years focused on the search for new species, on the determination of their chemical composition, and on the definition of their crystal structure [1,2,3,4,5,6,7,8,9,10]
Available genetic concepts on the origin of the low temperature hydrothermal vein-type selenium mineralization at El Dragón, which forms the subject of this paper, are distinct and consider only one stage of the ore-forming process
Förster et al [5] supposed that Se and the accompanying elements were transported within the same fluid, with Bi–Hg–Pb becoming progressively enriched in the fluid after formation of the main selenium minerals
Summary
Research on selenium minerals conducted in the past few years focused on the search for new species, on the determination of their chemical composition, and on the definition of their crystal structure [1,2,3,4,5,6,7,8,9,10]. Comparatively little effort was spent on studying the origin of their host mineralization, including implications on the source of the selenium and accompanying elements (e.g., [11]). This conclusion holds for the two selenium occurrences in the Andes of Bolivia: Virgen de Surumi ( known as “Pacajake”) [12,13], district of Hiaco de Charcas, Chayanta Province, and El. Dragón, Antonio Quijarro Province, both in the Department of Potosí [14]. Available genetic concepts on the origin of the low temperature hydrothermal vein-type selenium mineralization at El Dragón, which forms the subject of this paper, are distinct and consider only one stage of the ore-forming process. We provide a genetic model that considers the relationship between crystallization and deformation of primary and secondary minerals, and we discuss possible mechanisms for the origin and formation of that unique Se occurrence
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